The present invention relates to pulverized coal-fired furnaces and, more particularly, to improving the low load operation of fuel burners employed therein.
In view of today's fluctuating electricity demand, typified by peak demand occurring during weekday daytime hours and minimum demand occurring at night and on the weekends, electric utilities have chosen to cycle many of their conventional coal-fired steam generator boilers by operating them at full load during peak demand hours and reducing them to low loads during periods of minimum demand.
As a consequence of this mode of operation, the electric utilities have used large quantities of natural gas or oil to furnish additional ignition energy during low load operation because the current generation of coal-fired steam generator furnaces require stabilization of the coal flames when operating at low loads. The required amount of auxiliary fuel fired for stabilization pruposes is significant and, for example, to maintain a 500 megawatt coal-fired steam generator at 10 to 15 percent load during minimum demand periods would require the use of 450 gallons of oil per hour.
One common method of firing coal in conventional coal-fired steam generator boilers is known as tangential firing. In this method, pulverized coal is introduced to the furnace in a primary air stream through burners, termed fuel-air admission assemblies, located in the corners of the furnace. The fuel-air streams discharged from these burners are aimed tangentially to an imaginary circle in the middle of the furnace. This creates a fireball which serves as a continuous source of ignition for the incoming coal. More specifically, a flame is established at one corner which in turn supplies the required ignition energy to stabilize the flame emanating from the corner downstream of and laterally adjacent to it. When load is reduced, the flames emanating from each corner become shorter and, as a consequence, a reduction in the amount of ignition energy available to the downstream corner occurs. As a result, auxiliary fuel such as oil or natural gas must be introduced in each corner adjacent to the pulverized coal-air stream to provide additional ignition energy thereby insuring that a flameout and resultant unit trip will not occur.
Another problem associated with operating a coal-fired burner at low load results from the fact that the pulverizing mills typically operate with a fairly constant air flow over all load ranges. When furnace load is reduced, the amount of coal pulverized in the mills decreases proportionally while the amount of primary air used to convey the pulverized coal from the mills through the admission assemblies into the furnace remains fairly constant. Consequently, the fuel-air ratio decreases. When the load on the furnace is reduced to the low levels desired during minimum demand periods, the fuel-air ratio has decreased to the point where the pulverized coal-primary air mixture has become too fuel lean for ignition to stabilize without significant supplemental ignition energy being made available.
Accordingly, it is an object of the present invention to provide for stabilized ignition of pulverized coal flames in pulverized coal-fired steam generators operating at low load without firing auxiliary fuels such as natural gas or oil.